How do I create an IMAP account that connects to a local LAN IP address?

For a number of years I have used Outlook as my mail client but it is so awful I have been searching for a viable alternative. Having tried Mailspring I really like it but now have the problem of trying to connect to my mail archive, which is held on my local SYnology NAS drive, but connected via a local IP address. If I try to add the account in Mailspring, it requests an email address but doesn’t offer the IMAP server connection options I would normally expect to find. Does anyone have any suggestions as to how best to proceed?

Kind regards